This invention is directed to method and apparatus for depositing precursor onto a substrate and, more specifically, depositing a low vapor pressure copper precursor onto a silicon wafer using an ultrasonic nebulizer.
In the manufacture of semiconductor wafers and of other similarly manufactured articles, sequences of processes including coating, etching, heat treating and patterning are sequentially employed. Most of these processes involve the chemical or physical addition or removal of material to or from a surface of a substrate, usually transported as a vapor.
Certain coating processes in such sequences are preformed by chemical vapor deposition (CVD). CVD is preferred, for example, in applying films to the differently facing surfaces of holes through underlying layers, as, for example, to apply conductive films for the purpose of making interconnections across insulating layers and the like. The quality of the overall semiconductor wafer can be significantly affected by the integrity of the coating deposited during the CVD process. Therefore, great attention should be used during the precursor deposition step of the manufacturing process.
One typical apparatus for applying metallic coatings to semiconductor wafers, such as those made from silicon, includes a CVD reaction chamber to which liquid precursor is supplied via a mass flow controller. Prior to entering the CVD reaction chamber, the liquid precursor is fed through an atomizer which atomizes the liquid precursor as it enters the CVD reaction chamber. The atomization of the liquid precursor and subsequent evaporation into a gas phase is an essential step in the deposition process. To rapidly vaporize a low volatility liquid material into a gas phase, it is best to atomize the liquid into micron-size droplets first. This process increases the surface-to-volume ratio of the liquid precursor leading to increased evaporation rates.
Traditional atomizers use a carrier gas pressure differential in the vaporization process. This pressure differential can significantly decrease the temperature of the atomizing region because of adiabatic expansion. Consequently, the low temperature can slow down the evaporation process and even freeze the liquid precursor. Freezing precursor is especially problematic when depositing particular types of copper-based precursors onto a silicon wafer.
Another disadvantage of traditional atomizers is that they generate course droplets, i.e., greater than 100 microns. Because of their relatively large size, these droplets evaporate slowly with respect to their rate of travel through the vaporizer volume. The relatively large droplet size increases the probability of droplets colliding with each other or with the CVD reaction chamber wall before they evaporate completely. Collisions with the wall of the reaction chamber can lead to copper deposition onto the CVD reaction chamber walls. Collisions between droplets can lead to the combination of two smaller droplets into a single, larger droplet, further increasing the time for complete evaporation.
What is needed, therefore, is an apparatus and method for atomizing liquid precursors quickly and efficiently. More specifically, a liquid precursor atomization apparatus should atomize liquid precursor into small droplets to increase their evaporation rate.
The problems discussed above are overcome by the present invention which utilizes an ultrasonic nebulizer cooperating with a CVD reaction chamber in which semiconductor wafers, such as silicon wafers, are coated. The ultrasonic nebulizer converts high frequency electrical energy into mechanical displacement to precisely atomize small volumes of the liquid precursor into micron-size droplets. The ultrasonic nebulizer mounts directly onto the,cover member of the CVD reaction chamber, minimizing the distance that the vaporized precursor travels before reaching the silicon wafer onto which the precursor vapor is to be deposited. Minimizing the precursor travel distance also minimizes the opportunity for the precursor to contact other surfaces of the CVD reaction chamber which are not intended to receive the precursor deposition. Additionally, thermal convection from a heated sweeping gas assists in evaporating the precursor droplets atomized by the ultrasonic nebulizer. The heated sweeping gas replaces the conventional method of evaporation which relies upon the droplet impinging upon a heated surface which is at a temperature higher than the vaporization temperature for the liquid precursor. Preferably, the heated sweeping gas has a high heat capacity and low molecular weight such as hydrogen, helium, and argon. Use of these low molecular weight gases yields high binary diffusivity which greatly reduces evaporation time.
Therefore in accordance with the principles of the present invention a CVD apparatus for depositing a precursor onto a substrate includes a CVD reaction chamber with an interior containing a substrate holder adapted to support a substrate at a predetermined position within the CVD reaction chamber. An ultrasonic nebulizer is operatively connected to the CVD reaction chamber and is adapted to connect to a source of liquid precursor. The ultrasonic nebulizer has an atomizing discharge end adapted to atomize the liquid precursor and deposit the atomized precursor as a vapor onto a substrate supported by the holder. A gas distribution member is disposed within the interior of the CVD reaction chamber for discharging a directionally oriented gas into the atomized precursor to direct the atomized precursor toward the substrate. In one aspect of this embodiment, the directionally oriented gas is heated to further assist in the vaporization of the atomized precursor.
In one embodiment, a vaporization zone is provided around the ultrasonic nebulizer and the gas distribution ring. Specifically, side walls, a vaporizer plate, and the cover member of the CVD reaction chamber define the boundaries of the vaporization zone. The side walls particularly prevent any unvaporized precursor droplets from bypassing the vaporizer plate and possibly contacting the silicon wafer or the side walls of the CVD reaction chamber. Preferably, the vaporizer plate is heated to a temperature above the vaporization temperature of the particular precursor being deposited.
In another embodiment, the CVD reaction chamber includes a diffuser associated with the ultrasonic nebulizer and the gas distribution ring. The diffuser serves to maintain the atomized precursor and sweeping gas mixture with a uniform velocity front as it travels toward the silicon wafer. By properly choosing the diffuser geometry, the spread of the vaporized precursor is sufficient to cover the substrate surface without unfavorably contacting the CVD reaction chamber walls.
Various additional advantages, objects and features of the invention will become more readily apparent to those of ordinary skill in the art upon consideration of the following detailed description of the presently preferred embodiments taken in conjunction with the accompanying drawings.